1. Field of the Invention
The present invention relates to a suction pad suitable for attracting and conveying electronic components such as semiconductor devices for which it is necessary to avoid generation of static electricity, and to a rotation-stop and guide mechanism for a suction pad, with which the suction pad may be precisely guided to a workpiece to attract it, and while conveying the workpiece, rotation of the suction pad may be stopped to enable highly accurate positioning of the workpiece.
2. Description of the Related Art
A suction pad coupled with a vacuum source has been hitherto employed for conveying workpieces such as semiconductor devices. A suction pad is typically formed of electrically conductive rubber such as conductive NBR, conductive silicone rubber or the like.
A suction pad made of conductive rubber described has an inconvenience that when attracting a semiconductor device or the like, or when releasing it at a desired position, the surface of the semiconductor device subjected to attractive contact with the suction pad tends to bear static electricity due to friction, which possibly impairs the performance of the device.
Thus, a suction pad in which static electricity is prevented from generation on the surface of an attracted semiconductor device is disclosed, for example, in Japanese Patent Laid-open No. 4-82685.
As shown in FIG. 1, the suction pad 1 of the aforementioned laid-open patent has a metallic pad 2 fitted around a suction cup 3 made of conductive nitrile rubber which in turn is fixed to a metallic fixture 4. A gap G is provided between the leading end of the suction cup 3 and the surface of the electronic device 5 to be attracted, not to allow a direct contact therebetween.
With this arrangement, static electricity, that generated by friction between the metallic pad 2 and the device 5, and that borne in advance on the device 5, is discharged through the metallic pad 2, the suction cup 3 of nitrile rubber and the metallic fixture 4. However, the conductive nitrile rubber has a relatively high electrical resistance compared to metal, it is difficult to completely discharge the static electricity from the electronic device 5.
An all metal suction pad, as an alternative for the suction pad 1 shown in FIG. 1 for ease of discharging the static electricity, would suffer attraction failure, since such a suction pad lacks flexibility and makes a solid contact with the surface of the device 5. On the other hand, too fast discharge of the static electricity may damage the electronic device, depending on the type.
In an assembly line, an electronic device such as an IC chip laid in a pallet is attracted and transferred by a suction pad, to have the terminals of the IC chip inserted into predetermined holes of a board placed on the line so as to be electrically connected.
In conveying the IC chip in such a manner, there arises another inconvenience in which the suction pad is rotated to impact, vibration or other forces transmitted through a robot arm, or twist of a connection tube between a vacuum source and the suction pad produced from the movement of the robot arm. This makes it difficult to effect proper insertion of the IC chip terminals into the board holes, especially in a recent technical situation where high density surface mounting is required with IC chips becoming smaller and smaller. Small terminal distances of IC chips require highly accurate positioning of the IC chips on the board.
To attract and transfer a workpiece having an inclined surface, a suction pad associated with a bellows skirt as shown in FIGS. 2 and 3A has been hitherto employed. The bellows skirt 10 is usually made of electrically conductive rubber. When the skirted suction pad 6 attracts the workpiece 17, the bellows skirt 10 contracts as shown in FIG. 3B, which, after releasing the workpiece 17, normally re-assumes the original bellows shape shown in FIG. 3A.
However, sometimes, the skirt 10 fails to restore the original bellows shape, after releasing the workpiece 17, due to stickiness of the rubber material of the bellows, as shown in FIG. 3C. The restoration failure of the skirt 10 greatly impairs the adaptability of the skirt 10 to workpiece shapes, and the attracting capability of the suction pad 6.
Thus, as shown in FIGS. 4 and 5, a suction pad has been devised in which one outer wall surface of bellows is provided with a plurality of ribs 18a-18f protruding outwardly. The ribs 18a-18f prevent the bellows surfaces from sticking to each other. However, it has been experienced that a suction pad of this type has a tendency of being readily worn out at the outer wall surface of bellows to which the ribs contact repeatedly. Therefore, the problem of restoration failure in the skirted suction pad has not yet been finally solved.